The present invention relates to thermographic recording materials whose prints have improved image tone.
Thermal imaging or thermography is a recording process wherein images are generated by the use of thermal energy. In direct thermal thermography a visible image pattern is formed by image-wise heating of a recording material.
EP 692 733 discloses a direct thermal recording process wherein a direct thermal recording material is heated dot-wise and the direct thermal recording material comprises on a substrate an imaging layer containing uniformly distributed in a film-forming polymeric binder (i) one or more substantially light-insensitive organic silver salts being no double salts, the silver salt(s) being in thermal working relationship with (ii) an organic reducing agent therefor, wherein the reducing agent is a benzene compound the benzene nucleus of which is substituted by no more than two hydroxy groups which are present in 3,4-position on the nucleus and have in the 1-position of the nucleus a substituent linked to the nucleus by means of a carbonyl group.
EP-A 903 625 discloses a substantially light-insensitive black and white monosheet thermographic recording material is provided comprising a support and a thermosensitive element containing a substantially light-insensitive organic silver salt, a 1,2-dihydroxybenzene-compound in thermal working relationship therewith and a binder, characterized in that the 1,2-dihydroxybenzene-compound is represented by formula (I): 
where R is xe2x80x94P(xe2x95x90O)R1R2, xe2x80x94SOxR3, xe2x80x94CN, xe2x80x94NO2 or xe2x80x94CR4xe2x95x90NR5 when n is 0; R is xe2x80x94P(xe2x95x90O)R1R2, xe2x80x94SOxR3, xe2x80x94CN, xe2x80x94NO2, xe2x80x94CR4xe2x95x90NR5 or xe2x80x94COR6 when n is an integer; R1 and R2 are independently an alkyl, a substituted alkyl, an aryl, a substituted aryl group, an alkoxy, a substituted alkoxy, an aryloxy, a substituted aryloxy, a hydroxy group, an amino group or a substituted amino group; R3 is an alkyl, a substituted alkyl, an aryl, a substituted aryl, an amino or a substituted amino group; R4 is an alkyl, a substituted alkyl, an aryl or a substituted aryl group or hydrogen; R5 is an alkyl, a substituted alkyl, an aryl, a substituted aryl, a hydroxy, an alkoxy, an aryloxy, an acyl, an amino or a substituted amino group; R6 is an alkyl, a substituted alkyl, an aryl, a substituted aryl, an alkoxy, a substituted alkoxy, an aryloxy, a substituted aryloxy, a hydroxy, an amino or a substituted amino group or hydrogen; x is 1, 2 or 3; and the benzene ring of the 1,2-dihydroxybenzene-compound represented by the formula (I) may be further substituted.
Unpublished European Patent Application Nr. EP01000096.6 disclosed a monosheet black and white substantially light-insensitive thermographic recording material comprising a thermosensitive element and a support, the thermosensitive element containing a substantially light-insensitive organic silver salt, a 3,4-dihydroxybenzene compound in thermal working relationship therewith and a binder, characterized in that the 3,4-dihydroxybenzene compound is an aryloxo-3,4-dihydroxybenzene compound in which the aryl-group is substituted with at least one substituent having a "sgr"m-value greater than 0; or a heteroaryloxo-3,4-dihydroxybenzene compound in which the heteroaryl group has a unified aromaticity index IA greater than 53 and is optionally substituted with at least one group selected from the group consisting of aryl, hydroxy, carboxy, sulfo, sulfoalkyl, sulfoaryl, sulfonylalkyl, sulfonylaryl, annulated aryl, annulated heteroaryl, carboxyalkyl, carboxyaryl, oxoalkyl, oxoaryl, halogen, nitro, cyano and mercapto-alkyl groups; and a thermographic recording process therefor.
Unpublished European Patent Application Nr. EP01000095.8 disclosed a monosheet black and white substantially light-insensitive thermographic recording material comprising a thermosensitive element and a support, the thermosensitive element containing a substantially light-insensitive organic silver salt, a 1,2-dihydroxybenzene-compound in thermal working relationship therewith and a binder, characterized in that the 1,2-dihydroxybenzene-compound is represented by formula (I): R1SO2R2, wherein R1 is an optionally substituted aryl group and R2 is selected from the group consisting of a 3,4,5-trihydroxyphenyl group, a 3-alkoxy-4,5-dihydroxyphenyl group and a 3-aryloxy-4,5-dihydroxyphenyl group; or the 1,2-dihydroxybenzene-compound is represented by formula (II): R3COOR4, wherein R3 is a 3-alkoxy-4,5-dihydroxyphenyl group or a 3-aryloxy-4,5-dihydroxyphenyl group; and R4 is an alkyl group or an aryl group; and a thermographic recording process therefor.
U.S. Pat. No. 3,028,254 discloses heat-sensitive copy-sheets comprising a substantially light-insensitive organic silver salt, a reducing agent therefor and a binder. Specifically in Example 1 molar ratio of molar hydroxy-equivalents of reducing agents to molar silver-equivalents of the substantially light-insensitive organic silver salts of 1.78 and 1.33 are disclosed with respect to 3,3,3xe2x80x2,3xe2x80x2-tetramethyl-1,1xe2x80x2-spiro-bis-indane 5,5xe2x80x2,6,6xe2x80x2-tetrol and behenyl pyrogallol and silver behenate; and 3,3,3xe2x80x2,3xe2x80x2-tetramethyl-1,1xe2x80x2-spiro-bis-indane 5,5xe2x80x2,6,6xe2x80x2-tetrol and silver behenate respectively.
U.S. Pat. No. 3,031,329 discloses heat-sensitive copy-sheets comprising a substantially light-insensitive organic silver salt, a reducing agent therefor and a binder. Specifically in Examples 1 and 2 molar ratios of molar hydroxy-equivalents of reducing agents to molar silver-equivalents of the substantially light-insensitive organic silver salts of 7.29 and 4.13 are disclosed respectively with respect to 3,3,3xe2x80x2,3xe2x80x2-tetramethyl-1,1xe2x80x2-spiro-bis-indane 5,5xe2x80x2,6,6xe2x80x2-tetrol, 3,4-dihydroxybenzoic acid and silver behenate; and methyl gallate and silver behenate respectively.
U.S. Pat. No. 3,074,809 discloses heat-sensitive copy-sheets comprising a substantially light-insensitive organic silver salt, a reducing agent therefor and a binder. Specifically in Examples 1, 2 and 3 molar ratios of molar hydroxy-equivalents of reducing agents to molar silver-equivalents of the substantially light-insensitive organic silver salts of 5.725, 5.725 and 2.310 are disclosed respectively with respect to hydroquinone and silver behenate; hydroquinone and silver behenate; and methyl gallate, 2,3-dihydroxybenzoic acid and silver behenate respectively.
U.S. Pat. No. 3,103,881 discloses heat-sensitive copy-sheets comprising a substantially light-insensitive organic silver salt, a reducing agent therefor and a binder. Specifically in Example 3 a molar ratio of molar hydroxy-equivalents of reducing agents to molar silver-equivalents of the substantially light-insensitive organic silver salts of 2.95 is disclosed with respect to 3,4-dihydroxy-benzoic acid and silver behenate.
U.S. Pat. No. 3,107,174 discloses heat-sensitive copy-sheets comprising a substantially light-insensitive organic silver salt, a reducing agent therefor and a binder. Specifically in Example 1 a molar ratio of molar hydroxy-equivalents of reducing agents to molar silver-equivalents of the substantially light-insensitive organic silver salts of 3.29 is disclosed with respect to methyl gallate and silver behenate.
Typical X-ray images with conventional silver halide radiographic film have an image tone corresponding to CIELAB-values of a*=xe2x88x924.62 and b*=xe2x88x928.86 for D=1.0 and a*=xe2x88x922.53 and b*=xe2x88x924.41 for a density of 2.0 and exhibit excellent archivability both as regards stability of image density and stability of image tone. Prints produced with prior art thermographic recording materials which exhibit acceptable archivability exhibit a reddish image tone and prior art thermographic recording materials which exhibit clinically acceptable neutral (a*=b*=0) to bluish image tone (a*=0 to xe2x88x927; b* less than 0) exhibit unacceptable archivability. This problem is particularly acute if prints are produced with thermal head printers with a heating time xe2x89xa615 ms. It is desirable to obtain image tones as close to the image tone typically attained with silver halide radiographic film as possible.
It is therefore an aspect of the present invention to provide concepts to enable substantially light-insensitive thermographic recording materials to be developed whose prints exhibit both satisfactory archivability and image tone.
It is a further aspect of the present invention to provide concepts to enable substantially light-insensitive thermographic recording materials to be developed which upon printing with a thermal head printer with a heating time xe2x89xa615 ms produce prints exhibiting both satisfactory archivability and image tone.
Further aspects and advantages of the invention will become apparent from the description hereinafter.
It has been surprisingly found that with particular classes of 1,2-dihydroxybenzene compound reducing agents, an increase in the molar ratio of molar equivalents of active hydroxy groups in the 1,2-dihydroxybenzene compound reducing agent or mixtures thereof with respect to the molar equivalents of organic silver salt or mixtures thereof above the stoichiometric value of 1.00 had the unexpected result of reducing both the CIELAB a*-value of prints produced therewith without the expected prohibitive deterioration in the archival properties of the prints. This effect was found to be particularly pronounced when the prints were produced with a thermal head printer with heating time xe2x89xa615 ms.
The above mentioned aspects of the present invention are realized by providing a monosheet black and white substantially light-insensitive thermographic recording material comprising a thermosensitive element and a support, the thermosensitive element containing one or more substantially light-insensitive organic silver salts, one or more reducing agents consisting of one or more 1,2-dihydroxybenzene-compounds in thermal working relationship therewith and a binder, wherein the molar ratio of molar hydroxy-equivalents of the 1,2-dihydroxybenzene compounds to molar silver-equivalents of the substantially light-insensitive organic silver salts is between 1.2 and 6.0; the 1,2-dihydroxybenzene-compounds have a xe2x80x94(CHxe2x95x90CH)nR group in the 4 position wherein n is zero or an integer and R is a substituent with a Hammett "sgr"p constant  greater than 0.35 and  less than 0.95 exclusive of a carboxy-group; and the benzene ring of the 1,2-dihydroxy-compounds is optionally further substituted with an entity selected from the group consisting of an alkyl, substituted alkyl, alkenyl, aryl, heteroaryl, alkoxy, thioalkyl, aryloxy, thioaryl, thioheteroaryl, acyloxy, thioacyl, amido, sulphonamido and halogen groups, an annelated aryl ring system and an annelated heteroaryl ring system.
The above mentioned aspects of the present invention are also realized by providing a thermographic recording process for the monosheet black and white substantially light-insensitive thermographic recording material described herein comprising the steps of: (i) providing the thermographic recording material; (ii) bringing the thermographic recording material into the proximity of a heat source; (iii) applying heat imagewise from the heat source to the thermographic recording material; and (iv) removing the thermographic recording material from the proximity of the heat source.
Several embodiments are disclosed in the dependent claims.
According to a first embodiment of the thermographic recording process, according to the present invention, the heat source is a thermal head. According to a second embodiment of the thermographic recording process according to the present invention, the heat source is a thin film thermal head. According to a third embodiment of the thermographic recording material according to the present invention, the heat source is a thin film thermal head operating with heating time of xe2x89xa625 ms. According to a fourth embodiment of the thermographic recording material according to the present invention, the heat source is a thin film thermal head operating with heating time of xe2x89xa615 ms.
The term alkyl means all variants possible for each number of carbon atoms in the alkyl group i.e. for three carbon atoms: n-propyl and isopropyl; for four carbon atoms: n-butyl, isobutyl and tertiary-butyl; for five carbon atoms: n-pentyl, 1,1-dimethyl-propyl, 2,2-dimethylpropyl and 2-methyl-butyl etc.
xe2x80x9cAnnelatedxe2x80x9d means having a carbonxe2x80x94carbon bond in common with e.g. annelation of a benzene ring with a naphthalene ring results in anthracene or phenanthrene depending on which carbonxe2x80x94carbon bond in the naphthalene ring is common to both the naphthalene ring and the benzene ring.
By substantially light-insensitive is meant not intentionally light sensitive.
Selected values of Hammett "sgr"m and Hammett "sgr"p substituent constants are to be found in Advances in Linear Free Energy Relationships, Edited by N. B. Chapman and J. Shorter, published by Plenum Press, London in 1972 on pages 28-29. The highest value reported for the Hammett "sgr"m and Hammett "sgr"p substituent constants is regarded as being the Hammett "sgr"m and Hammett "sgr"p substituent constant for the purposes of the present invention.
The unified aromaticity index IA is described by C. W. Bird in Tetrahedron, 48(32), 335-340 (1992), which also discloses aromaticity index values for a large range of aromatic groups.
The molar hydroxy-equivalents of a 1,2-dihydroxybenzene compound is obtained by multiplying the molar concentration of the 1,2-dihydroxybenzene compound by the number of hydroxy-groups substituents on benzene rings in formula (I) and then adding these values together. For example ethyl 3,4-dihydroxybenzoate has two molar hydroxy-equivalents per mole, n-propyl gallate with the benzene ring substituted with three hydroxy-groups has three molar hydroxy-equivalents per mole and 3,3,3xe2x80x2,3xe2x80x2-tetramethyl-1,1xe2x80x2-spiro-bis-indane 5,5xe2x80x2,6,6xe2x80x2-tetrol with two benzene rings each substituted with two hydroxy-groups has four molar hydroxy-equivalents per mole.
The molar silver-equivalent of a substantially light-insensitive organic silver salt is obtained by multiplying the molar concentration of the substantially light-insensitive organic silver salt by the number of silver-atoms in the particular substantially light-insensitive organic silver salt e.g. silver behenate has one molar silver-equivalent per mole and silver adipate, being a silver salt of a dicarboxylic acid, has two molar silver-equivalents per mole.
The molar ratio of the molar hydroxy-equivalent of the 1,2-dihydroxybenzene compound to the molar silver-equivalent of the substantially light-insensitive organic silver salt is obtained by dividing the molar hydroxy-equivalent of the 1,2-dihydroxybenzene compound, obtained as described above, by the molar silver-equivalents of the substantially light-insensitive organic silver salt, obtained as described above.
If more than one 1,2-dihydroxybenzene compound and/or more than one substantially light-insensitive organic silver salt is present, the molar ratio of molar hydroxy-equivalents of the 1,2-dihydroxybenzene compounds to molar silver-equivalents of the substantially light-insensitive organic silver salts is determined by dividing the sum of the hydroxy-equivalents of the 1,2-dihydroxybenzene compounds present by the sum of the silver-equivalents of the substantially light-insensitive organic silver salts present.
The total line time of a thermal head is the time between the beginning of the printing of one line of pixels and the beginning of the printing of the next line of pixels in the printer transport direction. The total line time is equal to the active line time if no cooling time is included in the total line time. Should a cooling time be included in total line time, this cooling time should be subtracted from the total line time to obtain the active line time.
The heating time of a thermal head is obtained by multiplying the active line time of the thermal head by the ratio of the length of the resistance elements in the thermal head in the transport direction of the printer to the distance between the beginning of one printing line to the next printing line e.g. for an active line time of 12 ms, a resistance element length in the printer transport direction of 75 xcexcm and a distance between the beginning of one printing line to the next printing line of 50 xcexcm, the printing time is 12xc3x97(75/50)=18 ms. This heating time corresponds to the time during which the film element experiences heat.
Heating in association with the expression a substantially water-free condition as used herein, means heating at a temperature of 80 to 250xc2x0 C. The term xe2x80x9csubstantially water-free conditionxe2x80x9d as used herein means that the reaction system is approximately in equilibrium with water in the air, and water for inducing or promoting the reaction is not particularly or positively supplied from the exterior to the element. Such a condition is described in T. H. James, xe2x80x9cThe Theory of the Photographic Processxe2x80x9d, Fourth Edition, Macmillan 1977, page 374.
Molar Ratio of Molar OH-equivalents of the 1,2-dihydroxybenzene Compounds to Molar Ag-equivalents of the Organic Silver Salts
According to a first embodiment of the substantially light-insensitive thermographic recording material of the present invention the molar ratio of molar hydroxy-equivalents of the 1,2-dihydroxybenzene compounds to molar silver-equivalents of the substantially light-insensitive organic silver salts is 1.3 to 5.0.
According to a second embodiment of the substantially light-insensitive thermographic recording material of the present invention the molar ratio of molar hydroxy-equivalents of the 1,2-dihydroxybenzene compounds to molar silver-equivalents of the substantially light-insensitive organic silver salts is 1.3 to 4.0.
According to a third embodiment of the substantially light-insensitive thermographic recording material of the present invention the molar ratio of molar hydroxy-equivalents of the 1,2-dihydroxybenzene compounds to molar silver-equivalents of the substantially light-insensitive organic silver salts is 1.5 to 3.5.
According to a fourth embodiment of the substantially light-insensitive thermographic recording material of the present invention the molar ratio of molar hydroxy-equivalents of the 1,2-dihydroxybenzene compounds to molar silver-equivalents of the substantially light-insensitive organic silver salts is 1.5 to 3.0.
According to a fifth embodiment of the substantially light-insensitive thermographic recording material of the present invention the molar ratio of molar hydroxy-equivalents of the 1,2-dihydroxybenzene compounds to molar silver-equivalents of the substantially light-insensitive organic silver salts is 1.5 to 2.5.
According to a sixth embodiment of the substantially light-insensitive thermographic recording material of the present invention the molar ratio of molar hydroxy-equivalents of the 1,2-dihydroxybenzene compounds to molar silver-equivalents of the substantially light-insensitive organic silver salts is 1.3 to 2.0.
1,2-dihydroxybenzene Compounds According to Formula (I)
The 1;2-dihydroxybenzene-compounds used the substantially light-insensitive thermographic recording material of the present invention have a xe2x80x94(CHxe2x95x90CH)nR group in the 4 position wherein n is zero or an integer and R is a substituent with a Hammett "sgr"p constant  greater than 0.35 and  less than 0.95 exclusive of a carboxy-group; and the benzene ring of the 1,2-dihydroxy-compounds is optionally further substituted with an entity selected from the group consisting of an alkyl, substituted alkyl, alkenyl, aryl, heteroaryl, alkoxy, thioalkyl, aryloxy, thioaryl, thioheteroaryl, acyloxy, thioacyl, amido, sulphonamido and halogen groups, an annelated aryl ring system and an annelated heteroaryl ring system.
According to an seventh embodiment of the substantially light-insensitive thermographic recording material of the present invention the R group in the 1,2-dihydroxybenzene-compounds is xe2x80x94P(xe2x95x90O)R1R2, xe2x80x94SOxR3, xe2x80x94CN, xe2x80x94NO2, xe2x80x94CRxe2x95x90NR5 or xe2x80x94COR6; R1 and R2 are independently an alkyl, a substituted alkyl, an aryl, a substituted aryl group, an alkoxy, a substituted alkoxy, an aryloxy, a substituted aryloxy, a hydroxy group, an amino group or a substituted amino group; R3 is an alkyl, a substituted alkyl, an aryl, a substituted aryl, an amino or a substituted amino group; R4 is an alkyl, a substituted alkyl, an aryl or a substituted aryl group or hydrogen; R5 is an alkyl, a substituted alkyl, an aryl, a substituted aryl, a hydroxy, an alkoxy, an aryloxy, an acyl, an amino or a substituted amino group; R6 is a hydrogen atom or an alkyl, a substituted alkyl, an aryl, an aryl substituted with at least one substituent having a Hammett "sgr"m-constant  greater than 0 and  less than 0.85, an alkoxy, a substituted alkoxy, an aryloxy, a substituted aryloxy, a heteroaryl having a unified aromaticity index greater than 53 or a substituted heteroaryl group having a unified aromaticity index greater than 53; and x is 1, 2 or 3.
According to an eighth embodiment of the substantially light-insensitive thermographic recording material of the present invention the 1,2-dihydroxybenzene-compounds are selected from the group consisting of 3,4-dihydroxybenzoate alkyl and aryl esters, 3,4-dihydroxybenzophenone, 3,4-dihydroxybenzophenone compounds in which the benzene ring without hydroxy-group substituents is substituted with at least one substituent having a Hammett "sgr"m-constant  greater than 0 and  less than 0.85, 3,4-dihydroxy-acetophenone and 3,4-dihydroxybenzonitrile.
According to a ninth embodiment of the substantially light-insensitive thermographic recording material of the present invention, the 1,2 dihydroxybenzene compounds according to formula (I) are selected from the reducing agents disclosed in EP-B 692 733, EP-A 903 625 and unpublished European Patent Application Nr. EP01000096.6.
According to a tenth embodiment of the substantially light-insensitive thermographic recording material of the present invention, wherein the thermosensitive element contains more than one of the 1,2-dihydroxybenzene compounds according to formula (I) and one of the 1,2-dihydroxybenzene compounds according to formula (I) is 1,2-dihydroxybenzonitrile.
According to an eleventh embodiment of the substantially light-insensitive thermographic recording material of the present invention, the 1,2-dihydroxybenzene compounds according to formula (I) are 3,4-dihydroxybenzonitrile and 3,4-dihydroxybenzophenone.
According to a twelfth embodiment of the substantially light-insensitive thermographic recording material of the present invention, the 1,2-dihydroxybenzene compounds according to formula (I) is an alkyl 3,4-dihydroxybenzoate such as ethyl 3,4-dihydroxybenzoate and n-butyl 3,4-dihydroxybenzoate.
Suitable 1,2-dihydroxybenzene compounds according to the present invention are:
The thermosensitive element as used herein is that element which contains all the ingredients which contribute to image formation. According to the present invention, the thermosensitive element contains one or more substantially light-insensitive organic silver salts, one or more 1,2-dihydroxybenzene-compounds as reducing agents therefor in thermal working relationship therewith and a binder. The element may comprise a layer system in which the above-mentioned ingredients may be dispersed in different layers, with the proviso that the substantially light-insensitive organic silver salts are in reactive association with the reducing agents i.e. during the thermal development process the reducing agent must be present in such a way that it is able to diffuse to the particles of substantially light-insensitive organic silver salt so that reduction to silver can occur.
According to a thirteenth embodiment of the substantially light-insensitive thermographic recording material of the present invention, the organic silver salts are not double organic salts containing a silver cation associated with a second cation e.g. magnesium or iron ions.
According to a fourteenth embodiment of the substantially light-insensitive thermographic recording material of the present invention, at least one of the organic silver salts is a substantially light-insensitive silver salt of an organic carboxylic acid.
According to a fifteenth embodiment of the substantially light-insensitive thermographic recording material of the present invention, at least one of the organic silver salts is a substantially light-insensitive silver salt of an aliphatic carboxylic acids known as a fatty acid, wherein the aliphatic carbon chain has preferably at least 12 C-atoms, e.g. silver laurate, silver palmitate, silver stearate, silver hydroxystearate, silver oleate and silver behenate, which silver salts are also called xe2x80x9csilver soapsxe2x80x9d. Other silver salts of an organic carboxylic acid as described in GB-P 1,439,478, e.g. silver benzoate, may likewise be used to produce a thermally developable silver image. Combinations of different silver salt of an organic carboxylic acids may also be used in the present invention, as disclosed in EP-A 964 300.
Organic silver salts may be dispersed by standard dispersion techniques. Ball mills, bead mills, microfluidizers, ultrasonic apparatuses, rotor stator mixers etc. have been found to be useful in this regard. Mixtures of organic silver salt dispersions produced by different techniques may also be used to obtain the desired thermographic properties e.g. of coarser and more finely ground dispersions of organic silver salts.
Combinations of compounds according to formula (I) with a further reducing agent may also be used that on heating become reactive partners in the reduction of the substantially light-insensitive organic silver salt. According to an sixteenth embodiment of the substantially light-insensitive thermographic recording material of the present invention, the thermosensitive element contains a reducing agent in addition to 1,2-dihydroxybenzene compounds according to formula (I).
According to a seventeenth embodiment of the substantially light-insensitive thermographic recording material of the present invention, the thermosensitive element further contains at least one reducing agent disclosed in unpublished European Patent Application Nr. EP01000095.8, such as: 4-methyl-3xe2x80x2,4xe2x80x2,5xe2x80x2-trihydroxy-diphenylsulphone.
The film-forming binder of the thermosensitive element may be all kinds of natural, modified natural or synthetic resins or mixtures of such resins, in which the substantially light-insensitive organic silver salt can be dispersed homogeneously either in aqueous or solvent media: e.g. cellulose derivatives such as ethylcellulose, cellulose esters, e.g. cellulose nitrate, carboxymethylcellulose, starch ethers, galactomannan, polymers derived from xcex1, xcex2-ethylenically unsaturated compounds such as polyvinyl chloride, after-chlorinated polyvinyl chloride, copolymers of vinyl chloride and vinylidene chloride, copolymers of vinyl chloride and vinyl acetate, polyvinyl acetate and partially hydrolyzed polyvinyl acetate, polyvinyl alcohol, polyvinyl acetals that are made from polyvinyl alcohol as starting material in which only a part of the repeating vinyl alcohol units may have reacted with an aldehyde, preferably polyvinyl butyral, copolymers of acrylonitrile and acrylamide, polyacrylic acid esters, polymethacrylic acid esters, polystyrene and polyethylene or mixtures thereof.
According to an eighteenth embodiment of the substantially light-insensitive thermographic recording material of the present invention, the thermosensitive element contains a binder which does not contain additives or impurities which adversely affect the thermographic properties of the thermographic recording materials in which they are used.
According to a nineteenth embodiment of the substantially light-insensitive thermographic recording material of the present invention, the thermosensitive element further contains a so-called toning agent organic silver salt in order to obtain a neutral black image tone in the higher densities and neutral grey in the lower densities.
Suitable toning agents are described in U.S. Pat. No. 3,074,809, U.S. Pat. No. 3,446,648 and U.S. Pat. No. 3,844,797 and U.S. Pat. No. 4,082,901. Other particularly useful toning agents are the heterocyclic toning compounds of the benzoxazine dione or naphthoxazine dione type as disclosed in GB 1,439,478, U.S. Pat. No. 3,951,660 and U.S. Pat. No. 5,599,647.
According to a twentieth embodiment of the substantially light-insensitive thermographic recording material of the present invention, the thermosensitive element further contains one or more toning agents selected from the group consisting of phthalazinone, benzo[e][1,3]oxazine-2,4-dione, 7-methyl-benzo[e][1,3]oxazine-2,4-dione, 7-methoxy-benzo[e][1,3]oxazine-2,4-dione and 7-(ethylcarbonato)-benzo[e][1,3]oxazine-2,4-dione.
Stabilizers may be incorporated into the substantially light-insensitive thermographic recording materials of the present invention in order to obtain improved shelf-life and reduced fogging.
According to a twenty-first embodiment of the substantially light-insensitive thermographic recording material of the present invention, the thermosensitive element further contains at least one stabilizer selected from the group consisting of benzotriazole; substituted benzotriazoles; tetrazoles; mercaptotetrazoles, such as 1-phenyl-5-mercapto-tetrazole; and aromatic polycarboxylic acids, such as ortho-phthalic acid, 3-nitro-phthalic acid, tetrachlorophthalic acid, mellitic acid, pyromellitic acid and trimellitic acid, and anhydrides thereof.
According to a twenty-second embodiment of the substantially light-insensitive thermographic recording material of the present invention, the thermosensitive element further comprises at least one optionally substituted aliphatic (saturated as well as unsaturated aliphatic and also cycloaliphatic) polycarboxylic acid and/or anhydride thereof in a molar percentage of at least 15 with respect to all the organic silver salt(s) present and in thermal working relationship therewith. The polycarboxylic acid may be used in anhydride form or partially esterified form on the condition that at least two free carboxylic acids remain or are available in the heat recording step. According to a twenty-third embodiment of the substantially light-insensitive thermographic recording material of the present invention, the thermosensitive element further contains glutaric acid.
Surfactants and dispersants aid the dispersion of ingredients or reactants which are insoluble in the particular dispersion medium. The thermographic recording materials of the present invention may contain one or more surfactants, which may be anionic, non-ionic or cationic surfactants and/or one or more dispersants.
The recording material may contain in addition to the ingredients mentioned above other additives such as levelling agents e.g. BAYSILON(trademark) MA (from BAYER AG, GERMANY).
The support for the thermosensitive element according to the present invention may be transparent, translucent or opaque and is a thin flexible carrier made of transparent resin film, e.g. made of a cellulose ester, cellulose triacetate, polypropylene, polycarbonate or polyester, e.g. polyethylene terephthalate.
The support may be in sheet, ribbon or web form and subbed if need be to improve the adherence to the thereon coated thermosensitive element. It may be pigmented with a blue pigment as so-called blue-base. One or more backing layers may be provided to control physical properties such as curl and static.
According to a twenty-fourth embodiment of the substantially light-insensitive thermographic recording material of the present invention, the thermosensitive element is provided with a protective layer to avoid local deformation of the thermosensitive element and to improve resistance against abrasion.
According to a twenty-fifth embodiment of the substantially light-insensitive thermographic recording material of the present invention, the thermosensitive element is provided with a protective layer comprising a binder, which may be solvent-soluble, solvent-dispersible, water-soluble or water-dispersible.
According to a twenty-sixth embodiment of the substantially light-insensitive thermographic recording material of the present invention, the thermosensitive element is provided with a protective layer comprising solvent-soluble polycarbonates as binders, as described in EP-A 614 769.
According to a twenty-seventh embodiment of the substantially light-insensitive thermographic recording material of the present invention, the thermosensitive element is provided with a protective layer comprising a water-soluble or water-dispersible binder, as coating can be performed from an aqueous composition and mixing of the protective layer with the immediate underlayer can be avoided by using a solvent-soluble or solvent-dispersible binder in the immediate underlayer. The protective layer according to the present invention may be crosslinked. Crosslinking can be achieved by using crosslinking agents such as described in WO 95/12495. Solid or liquid lubricants or combinations thereof are suitable for improving the slip characteristics of the thermographic recording materials according to the present invention.
According to a twenty-eighth embodiment of the substantially light-insensitive thermographic recording material of the present invention, the thermosensitive element is provided with a protective layer comprising a solid thermomeltable lubricant such as those described in WO 94/11199. The protective layer of the thermographic recording material according to the present invention may comprise a matting agent. According to a twenty-ninth embodiment of the substantially light-insensitive thermographic recording material of the present invention, the thermosensitive element is provided with a protective layer comprising a matting agent such as described in WO 94/11198, e.g. talc particles, and optionally protrude from the protective layer.
According to a thirtieth embodiment of the substantially light-insensitive thermographic recording material, according to the present invention, the support, on the opposite side of the support to the thermosensitive element, is provided with an adhesion layer containing an intrinsically conducting polymer. The adhesion layer contains a binder e.g. a latex binder and a colloidal pigment e.g. colloidal silica.
According to a thirty-first embodiment of the substantially light-insensitive thermographic recording material, according to the present invention, the support, on the opposite side of the support to the thermosensitive element, is provided with an adhesion layer containing an intrinsically conducting polymer and the adhesion layer is provided with a backing layer optionally containing a second intrinsically conducting polymer. The backing layer contains a binder e.g. poly(vinyl alcohol), poly(methyl methacrylate) and gelatine, a pigment e.g. colloidal silica, and a matting agent e.g. silica particles or polymer particles e.g. poly(methyl methacrylate) particles.
According to a thirty-second embodiment of the substantially light-insensitive thermographic recording material, according to the present invention, the support, on the opposite side of the support to the thermosensitive element, is provided with an adhesion layer containing an intrinsically conducting polymer and the adhesion layer is provided with a backing layer optionally containing a second intrinsically conducting polymer and the intrinsically conducting polymer and/or the second intrinsically conducting polymer is a polythiophene.
According to a thirty-second embodiment of the substantially light-insensitive thermographic recording material, according to the present invention, the support, on the opposite side of the support to the thermosensitive element, is provided with an adhesion layer containing an intrinsically conducting polymer and the adhesion layer is provided with a backing layer optionally containing a second intrinsically conducting polymer and the intrinsically conducting polymer and/or the second intrinsically conducting polymer is a polythiophene, which is a polymer or copolymer of a 3,4-dialkoxythiophene in which said two alkoxy groups may be the same or different or together represent an optionally substituted oxy-alkylene-oxy bridge e.g. poly(3,4-ethylenedioxythiophene).
It is important that the pH of the dispersion for coating the outermost layer of the same side of the support as the thermosensitive element be less than 5 and substantially identical to that of the dispersion for coating the outermost layer of the side of the support opposite to that of the thermosensitive element. This results in a similar surface pH from the outermost layers on both sides of the support.
The coating of any layer of the recording material of the present invention may proceed by any coating technique e.g. such as described in Modern Coating and Drying Technology, edited by Edward D. Cohen and Edgar B. Gutoff, (1992) VCH Publishers Inc. 220 East 23rd Street, Suite 909 New York, N.Y. 10010, U.S.A.
Thermographic imaging is carried out by the image-wise application of heat either in analogue fashion by direct exposure through an image or by reflection from an image, or in digital fashion pixel by pixel either by using an infra-red heat source, for example with a Nd-YAG laser or other infra-red laser, with a substantially light-insensitive thermographic material preferably containing an infra-red absorbing compound, or by direct thermal imaging with a thermal head.
In thermal printing image signals are converted into electric pulses and then through a driver circuit selectively transferred to a thermal printhead. The thermal printhead consists of microscopic heat resistor elements, which convert the electrical energy into heat via Joule effect. The operating temperature of common thermal printheads is in the range of 300 to 400xc2x0 C. and the heating time per picture element (pixel) may be less than 1.0 ms, the pressure contact of the thermal printhead with the recording material being e.g. 200-500 g/cm2 to ensure a good transfer of heat.
In order to avoid direct contact of the thermal printing heads with the outermost layer on the same side of the support as the thermosensitive element when this outermost layer is not a protective layer, the image-wise heating of the recording material with the thermal printing heads may proceed through a contacting but removable resin sheet or web wherefrom during the heating no transfer of recording material can take place.
Activation of the heating elements can be power-modulated or pulse-length modulated at constant power. EP-A 654 355 discloses a method for making an image by image-wise heating by means of a thermal head having energizable heating elements, wherein the activation of the heating elements is executed duty cycled pulsewise. EP-A 622 217 discloses a method for making an image sing a direct thermal imaging element producing improvements in continuous tone reproduction.
Image-wise heating of the recording material can also be carried out using an electrically resistive ribbon incorporated into the material. Image- or pattern-wise heating of the recording material may also proceed by means of pixel-wise modulated ultra-sound.
Thermographic imaging can be used for the production of reflection type prints and transparencies, in particular for use in the medical diagnostic field in which black-imaged transparencies are widely used in inspection techniques operating with a light box.
The invention is illustrated hereinafter by way of comparative examples and invention examples. The percentages and ratios given in these examples are by weight unless otherwise indicated. The ingredients used in the substantially light-insensitive thermographic recording materials of the invention and comparative examples in addition to those disclosed above are given below:
Ingredients for the Backing and Adhesion Layers:
POVAL(trademark) 103=a 98% hydrolyzed poly(vinyl alcohol) from Kuraray;
V03/140=Erkol(trademark) V03/140, a 88% hydrolyzed poly(vinyl alcohol) from Acetex Europe;
KELZAN(trademark)S=a xanthan gum from MERCK and CO., Kelco Division, USA, which according to Technical Bulletin DB-19 is a polysaccharide containing mannose, glucose and glucuronic repeating units as a mixed potassium, sodium and calcium salt;
PE40=PERAPRET(trademark) PE40, a 40% aqueous dispersion of polyethylene latex from BASF;
Poligen(trademark) WE7=a 40% aqueous latex of oxidized polyethylene from BASF;
LATEX01=a 26.8% aqueous latex of a copolymer of 88% vinylidene chloride, 10% methyl acrylate and 2% itaconic acid and containing 0.13% Mersolat H;
LATEX02=a 20% aqueous poly(methyl methacrylate) latex with 100 nm particles;
LATEX03=a 30% latex of a copolymer of 88% vinylidene chloride, 10% methyl acrylate and 2% itaconic acid and 0.75% Hostapon(trademark) T;
LATEX04=a 20% aqueous poly(methyl methacrylate) latex with 50 nm particles;
PEDOT/PSS-1=a 1.2% aqueous dispersion of poly(3,4-ethylene-oxythiophene)/poly(styrene sulphonic acid) (1:2.46 by weight) produced as described in U.S. Pat. No. 5,354,613;
PEDOT/PSS-2=homogenized PEDOT/PSS-1;
Snowtex(trademark) O=a 20% aqueous dispersion of colloidal silica from Nissan Chemical;
Kieselsol 100F=a 30% aqueous dispersion of colloidal silica from BAYER;
UVONAC=a 10% aqueous solution of acetylated ULTRAVON W;
Mersolat(trademark) H=a 76% aqueous paste of a sodium pentadecyl-sulfonate from BAYER;
ZONYL(trademark) FSO 100=a block copolymer of polyethyleneglycol and polytetrafluoroethylene with the structure: F(CF2CF2)yCH2CH2O(CH2CH2O)xH, where x=0 to ca. 15 and y=1 to ca. 7 from DUPONT
Hostapon(trademark) T=a 40% concentrate of a sodium salt of N-methyl-N-2-sulfoethyl-oleylamide by HOECHST;
Arkopal NO 60=a nonylphenylpolyethylene-glycol from HOECHST;
OP80=Akypo(trademark) OP80, an 80% concentrate of an octyl-phenyl-oxy-polyethyleneglycol(EO 8)acetic acid from CHEMY;
MAT01=Sunsphere(trademark)51, a 8.63% dispersion of 5:7 xcexcm silica particles from Asahi Glass;
MAT02=a 20% aqueous dispersion of 6 xcexcm crosslinked beads of a copolymer of 98% methyl methacrylate and 2% stearyl methacrylate and 0.4% Arkopal NO 60 produced as described in U.S. Pat. No. 4,861,812;
Ingredients for Thermosensitive Element:
the organic silver salt:
AgB=silver behenate;
binders:
BL5HP=S-LEC BL5HP, a polyvinylbutyral from Sekusui;
crosslinking agent:
VL=Desmodur(trademark) VL, a 4,4xe2x80x2-di-isocyanatodiphenylmethane from BAYER;
the reducing agent:
CR01=methyl gallate (from U.S. Pat. No. 3,031,329 and U.S. Pat. No. 3,107,174);
CR02=3,3,3xe2x80x2,3xe2x80x2-tetramethyl-1,1xe2x80x2-spiro-bis-indane 5,5xe2x80x2,6,6xe2x80x2-tetrol (according to U.S. Pat. Nos. 3,028,254, 3,031,329 and EP 599 369): 
the toning agents:
T01=benzo[e][1,3]oxazine-2,4-dione;
T02=7-(ethylcarbonato)-benzo[e][1,3]oxazine-2,4-dione;
T03=7-methyl-benzo[e][1,3]oxazine-2,4-dione;
the stabilizers:
S01=glutaric acid;
S02=tetrachlorophthalic acid anhydride;
S03=benzotriazole.
Ingredients for the Protective Layers:
Ercol 48 20=Ercol(trademark) 48 20, a polyvinylalcohol from Acetex Europe;
26/88=MOWIOL(trademark) 26/88, a polyvinylalcohol from Clariant GmbH;
VP AC 4055=LEVASIL(trademark) VP AC 4055, a 15% aqueous dispersion of colloidal silica with a specific surface area of 500 m2/g from Bayer AG which had been converted into the ammonium salt;
ULTRAVON(trademark) W=a 75-85% concentrate of a sodium arylsulfonate from Ciba Geigy converted into acid form by passing through an ion exchange column;
SYLOID(trademark) 72=a silica from Grace;
VPDZ 3/100=SERVOXYL(trademark) VPDZ 3/100, a mono[isotridecyl poly-glycolether (3EO)] phosphate from Servo Delden B V);
VPAZ 100=SERVOXYL(trademark) VPAZ 100, a mixture of monolauryl and dilauryl phosphate from Servo Delden B. V.;
type P3=MICROACE(trademark) type P3, an Indian talc from Nippon Talc;
Satintone S=Satintone(trademark) S, a calcined china clay from Engelhard, USA;
RILANIT(trademark) GMS=a glycerine monotallow acid ester from Henkel AG;
TMOS=tetramethylorthosilicate hydrolyzed in the presence of methanesulfonic acid.